1. Field of the Invention
This invention relates to aqueous, acidic electrolytes containing nickel and cobalt ions and boric acid and also, as shine promotor, an aliphatic aldehyde and an aromatic carbonyl compound for the electrodeposition of hard, tarnish-resistant, white shining alloy coatings.
2. Statement of Related Art
The electrodeposition of metals on metallic surfaces for various purposes, for example, for improving wear resistance, friction reduction or corrosion prevention or even for decorative purposes, has been known for some time. Another known fact which has long been utilized in practice is that alloy coatings of several metals sometimes have more favorable properties for a certain application than individual coatings of the same metals. In particular, physical properties, such as coefficient of friction, temperature resistance or even magnetic permeability, can be specifically improved by coating a metal surface with an alloyed metal layer.
In numerous technological fields, good results have been obtained with nickel-cobalt alloys because not only are they characterized by favorable physical properties, they can also be readily electrodeposited in any desired composition. Electroplating baths containing suitable nickel and cobalt salts, for example chlorides, sulfates or sulfamates, boric acid and other additives influencing the electrical conductivity of the bath and the solubility of the anode, are used for forming nickel-cobalt alloy coatings.
On the other hand, German application No. 2,450,527 describes aqueous acidic baths for the electrodeposition of nickel coatings which, in addition to nickel salts and boric acid, also contain a so-called shine promotor consisting of an aliphatic C.sub.1 -C.sub.4 aldehyde and an aromatic compound containing a carbonyl group. However, nickel coatings such as these are attended by the disadvantage that, although their surface is light, it has a yellowish sheen, is sensitive to marring and tends to tarnish and has to be aftertreated, for example by chromium plating. In addition, the hardness of the electrodeposited coating is not always satisfactory.